Old criminal working in cloak (6)
I believe the answer is:
capone
'old criminal' is the definition.
(I know that Capone is a gangster)
'working in cloak' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'in' means one lot of letters goes inside another.
'cloak' becomes 'cape' (cape is a kind of cloak).
'on' placed inside 'cape' is 'CAPONE'.
(Other definitions for capone that I've seen before include "Al - , notorious gangster" , "Al ..., US gangster" , "Italian-American gangster, d. 1947" , "US gangster (St Valentine's Day massacre)" , "Hood" .)